1. Field of the Invention
The present invention generally concerns systems and methods for preventing or reducing the spillage of oil in the event of an accidental rupture of the tanks of a barge.
The present invention particularly concerns system and methods for creating, and for dynamically maintaining, a slight underpressure in the ullage space of the tanks of an unmanned barge which is generally towed by a tugboat.
2. Background Information
There are approximately 10,000 barges in the United States. As a group they are a principal polluter of inland waterways and coastal waters due to accidental ruptures in their hulls.
The Oil Pollution Act of 1990 mandates "Double Containment" for all barges less than 5,000 gross tons. "Double containment" within the meaning of the Act is not restricted to double hulls. Instead, it means any system that effectively prevent spillage of oil from an accidental rupture of the oil tanks of barges.
Barges under 5,000 gross tons are typically unmanned and without propulsion, i.e. they are not self propelled. The lack of these assets will be found to compatible to the present invention, which can be retrofitted to existing barges with minimum modifications.
2.1 Related Background to the Present Invention--Oil Spillage Avoidance
Avoidance of spillage from the rupture of tanks containing liquids--typically oil--is desirable on economic, environmental and aesthetic grounds. With the advent of supertankers, a single incidence of an oil spill from a large tanker can (i) cause significant damage to the environment, (ii) disrupt the ecological balance, and (iii) cause substantial economic loss. The accident of EXXON VALDEZ is perhaps the worst oil spillage disaster in U.S. history. The EXXON VALDEZ leaked about 240,000 barrels--over 10 million gallons of oil. The economic and environmental cost is estimated to have been over two billion dollars.
The predecessor, related, patent applications to the present application teach inventions for reducing or preventing any outflow of liquid, such as oil, due to the rupture of a tank, typically a ship's tank. The system, and method, of one related invention involves the creation, and the subsequent dynamic maintenance, of a partial vacuum in the effected tank or tanks. A partial vacuum below atmospheric pressure is preferably, and initially, created in the ship's tank before any rupture has occurred, and normally after a filling of the tank and before disembarkation of the ship. Thereafter the partial vacuum is continuously dynamically maintained in a precise balance responsive to the forces acting on the liquid contents of the tank, which forces change when the tank is ruptured. The dynamically maintained partial vacuum serves to hold the liquid contents of the tank within the tank even if, and when, the tank is ruptured--much in the manner that liquid is held within an inverted glass when the glass is pulled above the liquid level of a reservoir.
If the rupture is below the water line, and on the side of the ship's hull, then surface tension dynamics at the rupture between the tank's interior liquid, nominally oil, and the exterior water will induce a stratified flow, forcing water into the tank through the lower part of the rupture while forcing the liquid oil upward and out of the tank, oppositely to the flow of water. This stratified flow will continue until the water level reaches the top part of the rupture.
In one, preferred, embodiment of a related invention this stratified flow is stopped because a non-structural barrier, typically a tarpaulin, is placed over the rupture. The barrier is placed over the rupture even as, and while, the partial vacuum is dynamically maintained. The combination of dynamic underpressure control and the non-structural barrier substantially forestalls oil outflow--even below the level of the rupture.